1. Field of the Invention
The present invention relates to a liquid crystalline (LC) colorant polymer having improved orientation properties. In particular, the present invention relates to a LC colorant polymer wherein the improved orientation properties are due to lateral asymmetry in the colorant monomers used to prepare the copolymer.
2. Discussion of the Background
Liquid crystalline colorant polymers have been known for some time (see Ger. OS 30 27 571, and Ringsdorf, H., et al., in Chapoy, L.L., "Recent advances in liquid crystalline polymers", Elsevier, p. 253 (1985)). For example, colored polymers containing at least one mesogenic group and at least one group of a pleochroic colorant connected to the polymer chain via a spacer are described in Eur. OS 0,090,282. Additional liquid crystalline colorant polymers are described in U.S. Pat. No. 4,387,745, Brit. Pat. 2,193,338, and Ger. OS 3,825,066.
Liquid crystalline polymers having colorants copolymerized or mixed therein have attracted particular interest in connection with optical data storage media. In Ger. OS 38 37 936, polymers having laterally substituted mesogenic side chains are described as useful in a variety of products, such as optical structural elements for optical memories. The proposed systems are homo- and copolymers made from monomers of laterally alkyl-substituted mesogens. As the size of the lateral substituents increases, the nematic phase of the polymer becomes more stable, and the phase transition temperature is lower. But the described systems do not contain colorants.
According to Ger. OS 32 11 400, colorant-containing copolymers can be produced by copolymerization of the above-described system with comonomers containing colorant groups, but these groups are not specified. In Jap. 61-171,463, acrylic acid derivatives are described having side chains which contain mesogens comprised of azo dyes with a lateral methyl substituent. The phase behavior of the monomers and polymers is described briefly, and the product is proposed to be useful as a stationary material for gas chromatography. No additional physical properties are described.
The use of liquid crystalline colorant polymers for reversible optical data storage requires high sensitivity of the colorant molecule, short write and erase times, good long term stability, and a uniform macroscopic orientation of the colorant molecules in the storage medium.
Because the conventionally used azo dyes either have a smectic phase or are not mesogenic, copolymerizing these dyes with LC monomers provides a system which either displays smectic behavior at relatively high dye concentrations and low temperatures or has a strongly destabilized LC phase. In general, this severely limits the usable colorant concentrations. Since the smectic phase generally represents the low temperature phase having relatively high viscosity over a wide temperature interval, pre-orientation of the polymer samples in the low viscosity nematic phase must occur at relatively high temperatures in a generally narrow temperature interval. When the sample is cooled into the glass state, there is a transition through the smectic region resulting in substantial thermal stress on the polymer, and usually leading to appreciably inferior macroscopic orientation. The wider the temperature range of this phase, the more substantially the already achieved macroscopic orientation is reduced by reorientation phenomena.